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| United States Patent |
6,026,321
|
|
Miyata
, et al.
|
February 15, 2000
|
Apparatus and system for measuring electrical potential variations in
human body
Abstract
An apparatus for measuring potential variations in the human body,
comprising: a pair of conductors, an attachment patch for attaching the
conductors to the measurement sites; an amplifier that amplifies, as AC
signals, the potential variations input from the conductors through
connecting wires, a transmitter that transmits the amplified signals, a
battery that supplies electric power to the amplifier and transmitter, a
voltage-divider circuit that divides the voltage applied from the battery,
and a compensator circuit that applies a voltage so divided to the
amplifier as a reference voltage and, concurrently, rejects DC components
input to the conductors, using the divided voltage as a reference voltage.
Hence is provided a compact apparatus for measuring body signals such as
myoelectric potentials, without restricting the movements of the test
subject.
| Inventors:
|
Miyata; Takashi (Shizuoka, JP);
Matsuo; Noriyoshi (Shizuoka, JP);
Uchida; Hitoshi (Shizuoka, JP);
Sawada; Naomi (Shizuoka, JP);
Tomita; Yutaka (Kanagawa, JP)
|
| Assignee:
|
Suzuki Motor Corporation (Shizuoka, JP)
|
| Appl. No.:
|
050989 |
| Filed:
|
March 31, 1998 |
Foreign Application Priority Data
| Apr 02, 1997[JP] | 9-099762 |
| Dec 25, 1997[JP] | 9-367137 |
| Current U.S. Class: |
600/546; 128/903 |
| Intern'l Class: |
A61B 005/04 |
| Field of Search: |
600/546,407
128/903
|
References Cited
U.S. Patent Documents
| 3646606 | Feb., 1972 | Buxton et al.
| |
| 4522211 | Jun., 1985 | Bare et al.
| |
| 5168874 | Dec., 1992 | Segalowitz.
| |
| 5203330 | Apr., 1993 | Schaefer et al.
| |
| 5255677 | Oct., 1993 | Schaefer et al.
| |
| 5269302 | Dec., 1993 | Swartz et al.
| |
| 5318039 | Jun., 1994 | Kadefors et al.
| |
| 5368042 | Nov., 1994 | O'Neal et al.
| |
| 5450845 | Sep., 1995 | Axelgaard | 600/382.
|
| 5579781 | Dec., 1996 | Cooke | 128/903.
|
| Foreign Patent Documents |
| 62202805 | Dec., 1987 | JP.
| |
| 6349135 | Mar., 1988 | JP.
| |
| 2283354 | Nov., 1990 | JP.
| |
| 8196516 | Aug., 1996 | JP.
| |
Other References
"Precision, Low Power Instrumentation Amplifier", published in Mar. 1994 by
Burr-Brown Corporation.
"Development of an Active Electrode with an Amplifier for Surface
Electromyogram", by Suzushi Nishimura and Yutaka Tomita, Collected
Monographs of Automatic Measuring Control Society, vol. 29, No. 12, pp.
1474-1476 (1993) along with an English Abstract.
|
Primary Examiner: Kamm; William E.
Assistant Examiner: Layno; Carl H.
Attorney, Agent or Firm: Greenblum & Bernstein, P.L.C.
Claims
What is claimed is:
1. An apparatus for measuring electric potential variations in the human
body, comprising:
an attachment patch for attaching conductors to the measurement sites;
an amplifier for amplifying potential variations input from said conductors
through connection wires;
a transmitter for transmitting the amplified signals;
a battery for supplying electric power at a constant DC voltage to said
amplifier and transmitter;
a voltage-divider circuit to divide the voltage applied from said battery;
and
a compensator circuit that applies a voltage so divided to said amplifier
as a reference voltage and, the compensator circuit rejects DC components
input to said conductors, using said divided voltage as a reference
voltage;
said compensator circuit including an operational amplifier having an
output which directly feeds back to a negative terminal of said
operational amplifier, said output further being connected to both
positive and negative terminals of said amplifier.
2. The apparatus for measuring electrical potential variations in the human
body according to claim 1, wherein said voltage-divider circuit comprises:
two resistors of equal value connected, respectively, to the positive and
negative sides of said power supply; and
connecting wires for connecting between said two resistors and reference
terminal of said amplifier.
3. The apparatus for measuring electrical potential variations in the human
body according to claim 1, wherein said battery supplies electric power at
a voltage of 6 V to said transmitter, voltage-divider circuit, and
amplifier.
4. The apparatus for measuring electrical potential variations in the human
body according to claim 1, wherein said compensator circuit comprises a
voltage follower that negatively feeds back output to reference terminal
of said amplifier.
5. The apparatus for measuring electrical potential variations in the human
body according to claim 1, wherein said compensator circuit comprises:
resistors connected, respectively, to each of a pair of conductors; and
a connecting wire that makes an imaginary short at a prescribed voltage;
wherein said connecting wire is connected to:
reference terminal of said amplifier;
said resistor connected to one of said pair of conductors; and
resistor connected to other of said pair of conductors.
6. The apparatus for measuring electrical potential variations in the human
body according to claim 1 wherein said amplifier comprises an externally
attached resistor for setting gain; and a capacitor connected in series
with said resistor.
7. An apparatus for measuring electrical potential variations in the human
body, comprising:
a pair of conductor means;
attachment means for attaching said conductors to measurement sites;
amplifying means for amplifying potential variations input from said
conductors through conducting wires, as AC signals;
transmission means for transmitting amplified signals;
battery means for supplying electric power to said amplifier and
transmitter;
voltage-divider means for dividing voltage applied from said battery; and
compensator means that applies a voltage so divided to said amplifying
means as a reference voltage and that, concurrently, rejects DC components
input to said conductor means, using said divided voltage as a reference
voltage, said compensator means including an operational amplifier having
an output that feeds back directly into its negative terminal, and said
output being connected to positive and negative inputs of said amplifying
means.
8. The apparatus for measuring electrical potential variations in the human
body according to claim 7, wherein said compensator means comprising:
means for stabilizing the reference voltage to said amplifier;
means for rejecting DC components input to said conductors.
9. A system for measuring electrical potential variations in the human
body, comprising:
multiple electrometers that are secured to the skin and that transmit body
signals by radio; and
multiple receivers corresponding, respectively, to said electrometers;
wherein each of said electrometers comprises:
detection electrodes for inputting body signals;
a mixer-amplifier that mixes a reference signal of a constant voltage with
body signals input from said detection electrodes, said reference signal
having a frequency not contained in frequency band of those body signals,
and that amplifies these as a detected signal; and
a modulator-transmitter that modulates and transmits said detected signal
output from said mixer-amplifier with a carrier wave having a frequency
that is different for each of said electrometers; and
each of said receivers comprises:
a receiver-demodulator circuit that receives and restores said detected
signals from said electrometers; and
a filter circuit that separates said detected signals output from said
receiver-demodulator circuit into said body signals and said reference
signal.
10. The system according to claim 9, wherein said electrometers have
identifying labels that correspond with the frequency of the reference
signal for each electrometer; and said receivers have identifying labels
related to said identifying labels that correspond with said frequency of
said reference signal for each of said electrometers.
11. The system according to claim 9, further comprising a first plate on
the lower surface of which are provided detection electrodes and said
attachment patch; a second plate that is laminated to the upper surface of
said first plate and which becomes ground potential for said amplifier,
etc.; a third plate that is laminated to the upper surface of said second
plate and which accommodates electronic circuit board for said amplifier,
etc.; and a fourth plate that is laminated to the upper surface of said
third plate and which accommodates said battery.
12. The system according to claim 11, wherein said first through fourth
plates have circular flat surfaces.
13. The system according to claim 12, wherein said circular forms have a
diameter of 30-50 mm; and said first through fourth plates each have a
height of 0.7-15 mm.
14. The system according to claim 11, further comprising an antenna
connected to said transmitter circuit; and a cover that is provided at top
of said fourth plate and which protects said antenna.
15. An apparatus for measuring electrical potential variations in the human
body, comprising:
an attachment patch for attaching conductors to the measurement sites;
an amplifier for amplifying potential variations input from said conductors
through connection wires;
a transmitter for transmitting the amplified signals;
a battery for supplying electrical power at a constant DC voltage to said
amplifier and transmitter;
a voltage-divider circuit to divide the voltage applied from said battery;
a compensator circuit that applies a voltage so divided to said amplifier
as a reference voltage and, the compensator circuit rejects DC components
input to said conductors, using said divided voltage as a reference
voltage;
said voltage-divider circuit including two resistors of equal value
connected, respectively, to the positive and negative sides of said power
supply; and
said voltage-divider circuit including connecting wires for connecting
between said two resistors and a reference terminal of said amplifier.
16. An apparatus for measuring electrical potential variations in the human
body, comprising:
an attachment patch for attaching conductors to the measurement sites;
an amplifier for amplifying potential variations input from said conductors
through connection wires;
a transmitter for transmitting the amplified signals;
a battery for supplying electrical power at a constant DC voltage to said
amplifier and transmitter;
a voltage-divider circuit to divide the voltage applied from said battery;
a compensator circuit that applies a voltage so divided to said amplifier
as a reference voltage and, the compensator circuit rejects DC components
input to said conductors, using said divided voltage as a reference
voltage;
said compensator circuit comprising:
resistors connected, respectively, to each of a pair of conductors; and
a connecting wire that makes an imaginary short at a prescribed voltage;
wherein said connected wire is connected to:
a reference terminal of said amplifier;
said resistor being connected to one of said pair of conductors; and
a resistor connected to the other of said pair of conductors.
17. An apparatus for measuring electrical potential variations in the human
body, comprising:
an attachment patch for attaching conductors to the measurement sites;
an amplifier for amplifying potential variations input from said conductors
through connection wires;
a transmitter for transmitting the amplified signals;
a battery for supplying electrical power at a constant DC voltage to said
amplifier and transmitter;
a voltage-divider circuit to divide the voltage applied from said battery;
a compensator circuit that applies a voltage so divided to said amplifier
as a reference voltage and, the compensator circuit rejects DC components
input to said conductors, using said divided voltage as a reference
voltage;
wherein said amplifier comprises an externally attach resistor for setting
gain, and a capacitor connected in series with said resistor.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention concerns a human body signal measuring apparatus for
measuring, by radio, such human body signals as myoelectric potentials
(EMG=electromyography), oculo-electric potentials
(EOG=electro-oculography), cardio-electric potentials
(ECG=electrocardiography), and encephalo-electric potentials
(EEG=electroencephalography). This body signal measuring apparatus can be
employed not only in the field of medicine, but also in the fields of
physiology (including kinematics) and psychology (as in studies of
attention and arousal). It may also be attached to the driver of an
automobile or other moving vehicle to measure the driver's conditions.
2. Description of the Related Art
FIG. 13 is a circuit diagram for a conventional electromyograph. This
electromyograph will now be described with reference to this figure.
A conventional electromyograph 80 comprises a pair of detection electrodes
(conductor) 821 and 822 for inputting myoelectric potentials, a ground
electrode 832, an amplifier 84 for amplifying the myoelectric potentials
input from the detection electrodes 821 and 822, a lead wire 861 for
inputting the myoelectric potentials amplified by the amplifier 84 to a
measuring instrument 88, lead wires 862 and 863 for supplying power to the
amplifier 84, a lead wire 864 for supplying a reference voltage to the
amplifier 84, and a lead wire 865 for connecting the ground electrode 823
to ground potential.
The detection electrodes 821 and 822 are arranged concentrically in order
to measure myoelectric potentials without considering directionality. An
IC called the "AD620BR," made by Analog Devices, is used in the amplifier
84. A resistor 841 is connected to this IC for setting the gain. .+-.
power supplies 881 and 882 are built into the measuring instrument 88. The
.+-. power supplies supply electric power to the amplifier 84 via the lead
wires 862 and 863.
Myoelectric potentials may generally be thought of as AC signals referenced
to 0 V. Therefore, in order to measure the myoelectric potentials, .+-.
power supplies 881 and 882 are needed that are referenced to 0 V. In order
to stabilize this reference potential at 0 V, the ground potential is
applied to the skin in addition to the detection electrodes 821 and 822.
This ground electrode and its connecting lead wire 865 are connected to
the amplifier reference terminal. Thus the potential that constitutes the
reference for amplification is always 0 V. When a potential variation
having a DC component appears in the human body, this becomes a current
that flows to ground from the ground electrode 823. Even supposing then
that a DC component has been applied to the electrodes 821 and 822, that
DC component will be applied to the ground electrode 823 at that time,
whereupon the reference potential of the amplifier will also change. Thus
the amplifier will perform amplification normally.
An electromyograph such as this is set forth, for example, in Nishimura and
Tomita: "Zofuku kino wo motsu kindenikei no shisaku (Electromyograph with
amplification functions), " Keisoku Jido Seigyo Gakkai Ronbunshu
(Collected Monographs of Automatic Measuring Control Society), Vol. 29,
No. 12, pp. 1474-1476 (1993).
With the conventional electromyograph 80, five lead wires 861-865 are
required between it and the measuring instrument 88. Hence the movements
of the test subject are limited in range to the length of the lead wires
861-865. In other words, the movements of the test subject are limited to
simple, easy movements such as will not twist or pull out the lead wires
861-865.
For this reason, an electromyograph that operates by radio signals is
desirable. Unfortunately, however, electromyographs that operate on radio
signals are not known.
There are disclosures of techniques for transmitting heart rates by radio
signals, for example in the laid-open patent applications H2-283354 [1990]
and S63-49135 [1988]. These cardiotachometers can measure heart rate
without restricting the movements of the test subject, by adding radio
signal functions to the electrodes applied to the test subject. Compared
to myoelectric potentials or oculo-electric potentials, however, the
measurement of heart rate involves large potential differences, wherefore
there is little need to consider noise produced by DC components in the
human body.
Unlike cardiotachometry, when it comes to measuring such body potentials as
heart potential, myoelectric potential, oculo-electric potential, and
brain waves, the ground level has to be stabilized. When transmitting by
radio, in particular, it is necessary to amplify potential variations in
the human body. If the ground level is unstable, the amplifier will suffer
saturation or other impairment, preventing the realization of radio
transmissions.
In measuring myoelectric potentials, it is desirable to be able to measure
potential variation while the test subject is moving. Radio transmission
is thus most desirable in order to permit the test subject to move freely
and to prevent noise interference due to the shaking of lead wires. In
measuring myoelectric potentials, moreover, the smaller the electrodes are
made, the more muscle types one can take measurements from. Hence it is
desirable to do away with the ground electrode.
In view of these considerations, in designing an apparatus for measuring
fluctuations in human body potentials, the following problems need to be
resolved.
How to transmit the measured potential variations by radio in order to
eliminate the effects of lead wire shaking and allow the test subject to
move freely.
How to amplify body potential variations before transmission to facilitate
transmitting weak body potential variations by radio.
It is necessary to supply the amplifier with a stable reference potential
in order to amplify body potential variations. In order to do this
amplifying, however, the DC components in the body and the variations in
ground potential in the body need to be absorbed so that the amplifier
will only amplify the body potential variations (AC components), thereby
permitting a high amplification factor to be sustained.
Even if lead wires are eliminated so as not to restrict the movements of
the test subject, the test subject's movements will nevertheless be
restricted if the electrometer itself is large and/or heavy. In
particular, an electrometer is needed of such size as will not hinder
driving operations when attached to the driver of a motor vehicle.
In order to make the electrometer small, the battery needs to be limited to
a single cell, and the use of a ground electrode eliminated, while still
allowing the amplifier to function stably.
How to prevent crosstalk when multiple electrometers are attached to the
test subject in multiple places and radio transmissions are done
simultaneously, and how to maintain sufficient precision to permit
comparing the potentials detected by the several electrometers,
irrespective of differences in the performance of the electrometer
amplifiers.
SUMMARY OF THE INVENTION
An object of the present invention, therefore, is to provide an
electromyograph that is capable of measuring variations in weak body
potentials such as myoelectric potentials without restricting the
movements of the test subject.
Another object of the present invention is to detect body signals well
while rejecting the effects of noise such as body DC components, even with
a single battery cell, without having a common electrode.
Another object of the present invention is to stably maintain body signal
measurement precision even in configurations in which body signals are
amplified and transmitted by radio.
Another object of the present invention is to easily evaluate crosstalk
conditions between transmissions of body signals from multiple electrodes.
Another object of the present invention is to be able to eliminate
variation in the measurements of multiple electrometers, to make the
reference for the body signals transmitted from the electrometers
singular, and to make mutual comparisons of electrometer output values
with a constant reference.
The present invention, therefore, is an apparatus for measuring body
potential variation that comprises a pair of conductors, a bonding patch
for attaching the conductors to the measurement sites, an amplifier for
amplifying potential variations input from the conductors via connection
wires, as AC signals, a transmitter for transmitting the amplified
signals, a battery for supplying electric power to the amplifier and the
transmitter, a divider circuit for dividing the voltage applied from the
battery, and a compensator circuit for applying the a divided voltage as a
reference voltage to the amplifier and, at the same time, rejecting DC
components input to the conductors with the same divided voltage taken as
the reference. By these means is provided an apparatus that is small in
size for measuring body signals such as myoelectric potentials without
restricting the movements of the test subject.
Thus, by adopting a radio signal mode to replace the conventional wire
mode, lead wires are eliminated, and the test subject is allowed to move
freely.
In the prior art, two power supplies were used, namely a + and a - power
supply, and 0 V, or ground potential, in other words, was used as the
reference voltage. In the present invention, however, by dividing a single
constant DC voltage to obtain the reference voltage, one of the
conventionally needed two power supplies and the ground electrode are made
unnecessary. By amplifying the myoelectric potentials input from the
detection electrodes and outputting these as radio signals, moreover, the
lead wires required conventionally are rendered unnecessary, so the
movements of the test subject--formerly restricted by lead wires--are
freed up. In addition, the noise generated by the lead wires shaking or
functioning as receiving antennas can be eliminated.
By adding to the compensator circuit a function for rejecting myoelectric
potential noise input from the detection electrodes, moreover, the SN
ratio can be improved, so that even weak myoelectric potential signals can
be detected with greater precision than formerly.
When a plurality of electrometers is used simultaneously, furthermore, a
mixer for mixing a reference signal of a predetermined frequency not
contained in the body signal frequency band with the body signals input
from the detection electrodes may be beneficially employed. When this is
done, the frequency of the reference signal can be made different for each
electrometer, and the reference signal frequency separated by a filter
circuit will indicate a specific electrometer. Furthermore, if this
reference signal is made a constant voltage, then the ratio between the
voltages of the body signals and the reference signals separated by the
filter circuit will be such as to eliminate the effects of fluctuation in
amplification factor. This makes it possible to interpret potential
variations measured by multiple electrometers against the same reference.
When the frequency of the reference signal is made different for each
electrometer, in this manner, it is easy to verify which body signal is
coming in from which electrometer by the frequency of the reference signal
separated by the filter circuit in the receiver. It is also easy to
evaluate crosstalk between the several electrometers because the reference
signal frequency will be anomalous.
Furthermore, when the reference signal is made a constant voltage, it is
possible to obtain high-precision body signals from which the effects of
amplification factor fluctuation have been eliminated, by finding the
ratio between the voltages of the body signals and the reference signals
separated by the filter circuit.
When, moreover, the electrometers are specifically configured as laminated
bodies, as compared to when they are made flat, the area occupied can be
minimized, and the wiring distance between layers can also be minimized.
When this is done, the problems caused by noise produced by wire shaking
or wires functioning as antennas can be reduced. When measuring
myoelectric potentials, moreover, the number of muscle types from which
measurements are being taken can be increased by minimizing the areas
occupied.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a circuit diagram for a first embodiment of an electromyograph to
which the present invention pertains;
FIG. 2 is a circuit diagram for an example of an amplifier in the
electromyograph diagrammed in FIG. 1;
FIG. 3 is a circuit diagram for an example of a transmitter in the
electromyograph diagrammed in FIG. 1;
FIG. 4 is a simplified block diagram of a system for measuring body
signals;
FIG. 5 is an explanatory diagram showing an example of the system
diagrammed in FIG. 4 in use;
FIG. 6 is a circuit diagram of a sine wave oscillator circuit and a
mixer-amplifier circuit in the system diagrammed in FIG. 4;
FIG. 7 is a circuit diagram for an equivalent circuit from the signal
source to the amplifier in FIG. 6;
FIG. 8 is a diagonal view of one form of an electrometer;
FIG. 9 is a bottom view of the electrometer depicted in FIG. 8;
FIG. 10 is a vertical cross-sectional view in the VI--VI plane of FIG. 8;
FIG. 11 is a front elevation of the electrometer depicted in FIG. 8 but
with a cover provided;
FIG. 12 shows EMG waveforms derived by electrometer; and
FIG. 13 is a circuit diagram of a conventional electromyograph.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a circuit diagram for a first embodiment of an electromyograph to
which the present invention pertains; FIG. 2 is a circuit diagram for an
example of an amplifier in the electromyograph diagrammed in FIG. 1; and
FIG. 3 is a circuit diagram for an example of a transmitter in the
electromyograph diagrammed in FIG. 1. The present invention will now be
described with reference to these drawings. For the same elements as shown
in FIG. 13, however, the same reference characters are used, and redundant
descriptions are omitted.
The electromyograph 10 of this embodiment comprises a pair of detection
electrodes 821 and 822 that input myoelectric potentials, an attachment
patch 12 for securing the detection electrodes 821 and 822 to the skin, an
amplifier 13 that amplifies the myoelectric potentials input from the
detection electrodes 821 and 822 and outputs myoelectric potential
signals, a transmitter 15 that outputs the myoelectric potential signals
output from the amplifier 13, by radio, and a battery 16 that supplies
electric power to the transmitter 15 and the amplifier 13. The battery 16
outputs only one constant DC voltage. A compensator circuit 18 is
additionally provided for dividing a reference voltage from the constant
DC voltage and outputting it to the amplifier 13.
In the example diagrammed in FIG. 2, an IC called the "INA118," made by the
Burr-Brown Corporation, is used as the amplifier 13. To this IC is
connected a resistor 131 and a capacitor 132 in a series circuit. This
series circuit enables the realization of two functions, namely a function
for setting the gain when amplifying the myoelectric potentials input from
the detection electrodes 821 and 822, and the function of a high-pass
filter for rejecting noise contained in the myoelectric potentials input
from the detection electrodes 821 and 822 before they are amplified.
As diagrammed in FIG. 2, the amplifier 13 comprises a non-inverting
amplifier 142 and a differential amplifier 143. The non-inverting
amplifier 142 comprises an externally mounted resistor 131 and capacitor
132, operational amplifiers 133 and 134, and resistors 135 and 136, etc.
The differential amplifier 143 comprises resistors 137 and 138 and
operational amplifier 139. We express here the resistance of the resistor
131 by RG, the capacitance of the capacitor 132 by C, the resistance of
the resistors 135 and 136 by R1, the resistance of the resistors 137 and
138 by R2, and the resistance of the resistors 140 and 141 by R3. Resistor
131 is for setting the gain. If the capacitor 132 is not present, that is,
if the resistor 131 is shorted, then the gain G is given by G=1+50
(K.OMEGA.)/RG.
In this embodiment, the capacitor 132 is connected in series with the
resistor 131, so the gain G is given by the following formula.
G={(RG+2R1)j.omega.C+1}/(j.omega.CRG+1)(R3/R2) (Formula 1)
Here, if we let R3=R2, RG<<R1, and .omega.CR>>1, then Formula 1 reduces to
Formula 2.
.vertline.G.vertline.=1+2R1/RG(.omega..fwdarw..infin.) (Formula 2)
Then the cutoff frequency fC is given by formula 3.
fC=1/2.pi.CRG (Formula 3)
Referring once again to FIG. 1, the compensator circuit 18 comprises
resistors 181 and 182 for dividing the constant DC voltage n V (which may
be 3 V or 6 V, for example) of the battery 16, an operational amplifier
183 to function as a voltage follower for outputting the reference voltage
n/2 V (which may be 1.5 V or 3 V, for example) obtained from the resistors
181 and 182 to the amplifier 13, and resistors 184 and 185 connected
between the output terminal of the operational amplifier 181 and the .+-.
input terminals of the amplifier 13, respectively. By connecting the
resistors 184 and 185, respectively, between the output terminal of the
operational amplifier 183 and the .+-. input terminals of the amplifier
13, the voltages from the detection electrodes 821 and 822 to the .+-.
input terminals of the amplifier 13 are negatively fed back to the
operational amplifier 183 via the resistors 184 and 185. Hence the noise
is removed from the myoelectric potentials input from the detection
electrodes 821 and 822. The rejected noise comprises DC components and
common mode noise. The resistors 181, 182, 184, and 185 may have values,
respectively, of 1 M.OMEGA., for example.
When the battery has a voltage of 6 V, the voltage is 6 V at point e, so
power will be supplied to the transmitter 15, operational amplifier 183,
and amplifier 13 at a voltage of 6 V. Meanwhile, this voltage is divided
by the resistors 181 and 182, so 3 V will appear at point c. The
operational amplifier 183 is feeding back a negative output, so it will
function to constantly maintain this voltage at 3 V. Thus the output of
the amplifier 13 to the reference terminal and the positions where the
resistors 184 and 185 are connected will always be at 3 V. The wire
connecting this electrode and the amplifier is connected to the
operational amplifier 183 through the resistors 184 and 185. For this
reason, the reference voltage for the detection electrodes 821 and 822
will be the divided 3 V. An AC signal referenced against this 3 V is input
to the amplifier 13.
Since the connecting wires between these electrodes and the amplifier are
connected to the operational amplifier 183 through the resistors 184 and
185, the AC component will exhibit only very slight corruption, depending
on the value of the resistors, and will not be input on the operational
amplifier side. Meanwhile, the DC components above 3 V that appear on the
electrodes 821 and 822 are shunted immediately to ground at point f by the
negative feedback of the operational amplifier. Thus by dividing the
voltage before inputting to the operational amplifier, a single battery
may suffice. Also, because the detection electrodes 821 and 822 are
connected to the operational amplifier through the resistors 184 and 185,
the DC components on the detection electrodes are not input to the
amplifier 13. And, by making the voltage-divided op-amp output the
reference voltage for the amplifier, and, at the same time, the reference
voltage for the detection electrodes, even if a DC component is input to
the amplifier, the reference voltage will change simultaneously, wherefore
that DC component will not become noise. Because this operational
amplifier is provided with negative feedback, moreover, both the reference
voltage to the amplifier 13 and the reference voltage for the detection
electrodes will stabilize. By combining the operational amplifier 183 and
the resistors 184 and 185 together with the voltage-divider circuit made
up of the resistors 181 and 182, not only is a ground electrode rendered
unnecessary, but it is possible to amplify body potential variations
stably with a high amplification factor, without degrading the
amplification factor by the effects of DC components and other common mode
noise. Since it is possible, then, to amplify stably with a high
amplification factor, the signals detected can be transmitted by radio.
The body potential difference becomes the potential difference between the
points marked c and d, which is input to the transmitter 15 as an AC
signal.
The transmitter 15, as diagrammed in FIG. 3, may be an ordinary
frequency-modulated (FM) oscillator circuit comprising a high-pass filter
section 148 and an FM oscillator circuit section 149. The high-pass filter
section 148 comprises a capacitor 150 and resistors 151-153. The FM
oscillator section 149 comprises resistors 154 and 158, capacitors 155 and
160, a variable capacitor 156, and a coil 159. The capacitance of the
variable capacitor 156 is varied to regulate the oscillation frequency. A
wireless module in the "HRF-260 series" made by Hertz Radio Company, for
example, may be used as the transmitter 15.
The attachment patch 12 may be an adhesive applied between the detection
electrodes 821 and 822, for example.
The operation of the electromyograph 10 is next described. When the
detection electrodes 821 and 822 are secured to the skin by the attachment
patch 12, the myoelectric potentials produced by muscle activity are input
from the detection electrodes 821 and 822. The myoelectric potentials so
input are amplified by the amplifier 13, and then output to a measuring
instrument (not shown) by radio signal from the transmitter 15. Thus the
conventionally required lead wires are not needed, so that the test
subject has freedom of movement instead of being restricted by the lead
wires. Also, since constant DC voltage is divided and used as the
reference voltage, by means of the battery 16 and compensator circuit 18,
one of the two batteries conventionally required, and the ground
electrode, are also made unnecessary. As a consequence, the
electromyograph 10 can be implemented in a small light-weight package.
An embodiment is next described in which multiple electrometers 10 are
employed to simultaneously measure potential variations at a plurality of
sites on the test subject.
With reference to FIG. 4 and 5, the system for measuring body potential
differences, in this embodiment, is a system for measuring myoelectric
potentials, comprising four electrometers 121-124, secured to the skin of
a test subject M, for sending body signals by radio, and four receivers
141-144 corresponding, respectively, to the four electrometers 121-124.
The signals output by the receivers 141-144 are input to an analyzer unit
160. The analyzer unit 160 may be a computer.
The electrometer 121 comprises detection electrodes 822 and 821 for
inputting body signals S1, a mixer-amplifier 22 that mixes a reference
signal C1 having a frequency not included in the frequency band of the
body signals S1 with the body signals S1, amplifies these, and outputs a
detected signal D1, and a modulating transmitter 24 that modulates the
detected signal D1 output from the mixer-amplifier 22 with a carrier wave
FM1 that is of a different frequency for each of the electrometers 121-124
and transmits these. The receiver 141 comprises a receiver-demodulator
circuit 26 that receives and demodulates the detected signal D1
transmitted from the electrometer 121, and a filter circuit 28 that takes
the detected signal D1 output from the receiver-demodulator circuit 26 and
separates it into body signals S1 and a reference signal C1.
The reference signal C1 has a different frequency for each of the
electrometers 121-124, and is at a constant voltage. The electrometers
122-124 have the same configuration as the electrometer 121 excepting that
the frequencies of the reference signals C2-C4 (not shown) and the carrier
waves FM2-FM4 are different. The receivers 142-144 have the same
configuration as the receiver 141 excepting that the frequencies just
mentioned are different.
The detection electrodes 822 and 821 are arranged concentrically so that
the myoelectric potentials can be measured without worrying about
directionality. The mixer-amplifier 22 comprises a sine wave generator
oscillator circuit 30 that generates the reference signal C1, and an
amplifier 32 that mixes the reference signal C1 with the body signals S1,
amplifies these, and outputs them as the detected signal D1. The
modulator-transmitter 24 and the receiver-demodulator circuit 26 are
ordinary devices comprising ICs for frequent modulation and frequency
demodulation, respectively. The filter circuit 28 comprises a high-pass
filter 281 that separates the reference signal C1 from the detected signal
D1, and a low-pass filter 282 that separates the body signals S1 from the
detected signal D1.
FIG. 6 is a circuit diagram of a sine wave generating oscillator circuit
and a mixer-amplifier in the body signal measuring apparatus diagrammed in
FIG. 4. FIG. 7 is a circuit diagram of an equivalent circuit from the
signal source to the amplifier in FIG. 6. Further description will now be
given, making reference to these drawings.
The mixer-amplifier 32 comprises an amplifier 34 and a compensator circuit
36. The power supply voltage Vcc is supplied from a battery (not shown).
This is a single constant DC voltage. The compensator circuit 36,
comprising an operational amplifier 361 and resistors 362-365, divides the
supply voltage Vcc and outputs the voltage so divided to the amplifier 34
and operational amplifier 361. The compensator circuit 36 and amplifier 34
are configured as indicated in FIG. 1.
The sine wave oscillator circuit 30 comprises a signal source 301 that
generates a sine wave that becomes the basis of the reference signal C1,
and an amplitude attenuator circuit 302 that attenuates the sine wave
generated by the signal source 301 to a prescribed amplitude. The signal
source 301 may be an ordinary Wein bridge circuit, quadrature circuit, or
crystal oscillator circuit, etc. The amplitude attenuator circuit 301
comprises a capacitor 311 and resistors 312-314.
The input impedances at the .+-. input terminals of the amplifier 34 both
need to be made the same in order to avoid noise interference due to a
drop in the CMRR (common mode rejection ratio). Accordingly, if we
designate the values of the resistors 364, 365, 312, 313, and 314,
respectively, as r1, r2, r3, r4, and r5 (where r5>>r4), then the
resistance values are selected so as to satisfy the following
relationship.
r1={r2.times.(r3+r4)}/{r2+(r3+r4)} (Formula 4)
If the output voltage from the signal source 301 is designated E1, then the
input voltage E2 on the--input terminal of the amplifier 34 will be given
by Formula 5.
E2=r2/{(r4//r5+r3)+r2}].times.{r4+(r4+45)}.times.E1 (Formula 5)
By mixing this reference signal with the body signals, it becomes possible
to evaluate the amplitudes of body signals measured by multiple
electrometers using the same reference.
The operation of the body signal measuring apparatus 10 is next described,
making reference to FIG. 4 and 5.
The electrometer 121 is first described. When the electrometer 121 is
secured to the skin, body signals S1 constituted by myoelectric potentials
that appear as a result of muscle activity are input from the detection
electrodes 822 and 821. The body signals S1 so input are mixed with the
reference signal C1 and amplified by the mixer-amplifier 22 to form the
detected signal D1. This detected signal D1 is transmitted from the
modulator-transmitter 24 on the carrier wave FM1 to the receiver 141 where
it is restored by the receiver-demodulator circuit 26.
Here, if we designate the voltage and frequency of the body signals S1 as
Vs1 and Fs1, respectively, and the voltage and frequency of the reference
signal C1 as Vc1 and Fc1, respectively, the detected signal D1 will have a
voltage of .alpha.(Vs1+Vc1) and the frequency components (Fs1, Fc1). If
for the electrometers 122-124, similarly, we designate the voltages and
frequencies of the body signals S2-S4 as Vs2-Vs4 and Fs2-Fs4,
respectively, and the voltages and frequencies of the reference signals
C2-C4 as Vc2-Vc4 and Fc2-Fc4, respectively, the detected signal D2-D4 will
have voltages of .alpha.(Vs2+Vc2)-.alpha.(Vs4+Vc4) and frequency
components of (Fs2, Fc2)-(Fs4, Fc4), respectively. The body signals S1-S4
are myoelectric potentials, so the voltages will be in the mV range, and
the frequencies from 5 Hz to 1 kHz. For the reference signals C1-C4, for
example, we may have voltages Vc1-Vc4=10 mV, and frequencies of Fc1=2.0
kHz, Fc2=2.5 kHz, Fc3=3.0 kHz, and Fc4=3.5 kHz.
In the filter circuit 28 in the receiver 141, the cutoff frequency cf for
the high-pass filter 281 and the low-pass filter 282 is selected so as to
satisfy the relationship Fsmax<cf<Fcmin, where Fcmin is the minimum
frequency of the reference signals C1-C4 and Fsmax is the maximum
frequency of the body signals S1-S4. Accordingly, the voltage .alpha.Vc1
and the frequency Fc1 will be obtained from the high-pass filter 281, and
the voltage .alpha.Vs1 and the frequency Fs1 will be obtained from the
low-pass filter 282. The cutoff frequency cf will have the same values
also in the receivers 142-144.
Now, the frequency Fc1 obtained from the high-pass filter 281 will indicate
the electrometer 121, making it easy to verify that body signals S1 coming
in from the electrometer 121 are being received. And, if the carrier waves
FM2 and so on from the other electrometers 122 and so on appear as
crosstalk on the carrier wave FM1 of the electrometer 121, this will
result in a beat note with the mixing of the other frequencies Fc2 and so
on with the frequency Fc1, so that crosstalk can be easily evaluated. In
the event that crosstalk occurs as a result of setting the carrier
frequency of the electrometer 121, etc., in error, that, similarly, can be
easily evaluated.
In addition, from the voltage .alpha.Vs1 obtained from the low-pass filter
282, the voltage .alpha.Vc1 obtained from the high-pass filter 281, and
the known voltage Vc1 of the reference signal C1, the voltage Vs1 of the
body signal S1 can be calculated, using Formula 6.
Vs1=Vc1.times.(.alpha.Vs1)/(.alpha.Vc1) (Formula 6)
As is evident from Formula 6, the effects of fluctuations in the
amplification factor .alpha. have been eliminated from the value of the
voltage Vs1.
The morphology of the electromyograph is next described, making reference
to FIG. 8-11. FIG. 8 is a diagonal view thereof, FIG. 9 a bottom view,
FIG. 10 a vertical cross-sectional view in the VI--VI plane in FIG. 8, and
FIG. 11 a diagram of the configuration when an antenna is attached.
The electrometer 20 in this embodiment comprises a first plate 22 on the
lower surface of which are provided detection electrodes 821 and 822 and
an attachment patch 12, a second plate 24 that is laminated to the upper
surface of the first plate 22 and which becomes ground potential for the
amplifier 13, transmitter 15, and compensator circuit 18, a third plate 26
that is laminated to the upper surface of the second plate 24 and in which
is accommodated the electronic circuit board for the amplifier 13,
transmitter 15, and compensator circuit 18, and a fourth plate 28 that is
laminated to the upper surface of the third plate 26 and in which is
accommodated the battery 16. The diameters of all of the circular plates
22, 24, 26, and 28 are the same. The plates 22 and 24 are provided with
through holes 301 and 302 through which are passed lead wires 321 and 322
for connecting the detection electrodes 821 and 822 and the amplifier 13.
With the plates 22, 24, 26, and 28 laminated into a four-ply structure, the
area occupied is minimized, as compared to when a flat configuration is
used, and the wiring distances between the layers are also minimized. As a
result, the electrometer 20 can be made both small and light in weight.
Also, when the area occupied is minimized, measurements can be taken from
more types of muscles.
As depicted in FIG. 11, a cover 25 can also be laminated on to protect the
transmitter antenna 27. When that is done, there is no danger of the
antenna getting caught on something when the test subject moves around.
This cover will have a diameter of from 3 to 5 cm and a height of about 1
cm. In the example in which the frequency of the reference signal is
altered for each electrometer, a number can be applied to the antenna
cover 25 to serve as a reference corresponding to the number of the
receiver.
In the example depicted in FIG. 11, no switch is provided. By configuring
the device so that it is charged without providing a switch, a smaller
size and lighter weight can be realized.
FIG. 12 shows EMG waveforms derived by the wireless electrodes on jumping
movement (a) right gastroenemius muscle, (b) left gastrocnemius muscle,
(c) right tibialis anterior muscle. In FIG. 12, base voltage is change
from 3 V to 0 V for display.
The invention may be embodied in other specific forms without departing
from the spirit or essential characteristic thereof. The present
embodiments is therefore to be considered in all respects as illustrative
and not restrictive, the scope of the invention being indicated by the
appended claims rather than by the foregoing description and all changes
which come within the meaning and range of equivalency of the claims are
therefore intended to be embraced therein.
The entire disclosure of Japanese Patent Application No. 9-099762 (filed on
Apr. 2, 1997) and 9-367137 (filed on Dec. 25, 1997) including
specification, claims, drawings and summary are incorporated herein by
reference in its entirety.
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